Air cycle machines are often used in aircraft systems, such as in aircraft air conditioning systems, aircraft pressurization systems, etc. An air cycle machine includes an air bearing that uses a film of pressurized air to provide a low-friction load-bearing interface between two surfaces. At least one of the two surfaces may be rotating at high speed. The pressurized air prevents the two surfaces from touching each other thereby allowing the rotating surface to rotate freely. However, when an air bearing becomes overloaded, the surfaces can touch, leading to abnormal speed fluctuations of the rotating surface. Failure of the air bearing can lead to substantially constant contact between the two surfaces, which can cause materials from the surfaces to rub off and be emitted into the air. Such air emissions can be transmitted into a cabin of the airplane, requiring the aircraft to land. Thus, there is a need to be able to monitor the air cycle machine in order to be able to shut down the air cycle machine before such an event occurs.